FIG. 1 shows a circuit comprising an input terminal and an output terminal, a transistor having a load path and a control terminal, the load path of which is connected between the input terminal and output terminal, and a driver circuit which is connected to the control terminal of the transistor. A circuit arrangement of this type is described, for example, in EP 0 990 199 B1 as well as FIG. 1. This circuit arrangement has an input terminal IN to apply an input voltage Vin, an output terminal OUT to supply an output voltage Vout for a load Cout, and a bipolar transistor Q11 interconnected between input and output terminals IN, OUT. To drive transistor Q11, there is a driver circuit which is connected to the base terminal of transistor Q11.
A circuit arrangement of this type finds application, for example, in voltage regulators which supply a regulated output voltage at the output terminal from an input voltage applied at the input terminal, or in current regulators which supply a predefined current from the input terminal to the output terminal.
In this arrangement, the current from the input terminal to the output terminal IN, OUT, is controlled by the driver circuit 3 through the base current of transistor Q11. In a voltage regulator; driver circuit 3 is designed to control the base current of transistor Q11 as a function of the output voltage Vout applied at the output terminal—as described, for example, in the above-mentioned EP 0 990 199 B1. In a current regulator, the driver circuit controls the base current as a function of the current which flows from the input terminal to the output terminal.
In FIG. 1, the circuit arrangement with capacitive load Cout is wired to output terminal OUT of the load. The input voltage is supplied, for example, by a battery, not shown here. A resistance connected between input terminal IN and the base of the transistor prevents, in a known manner, the transistor to turn on at high temperatures due to leakage currents. Such resistors are known as “anti leakage” resistors.
In a circuit of this type, problems can occur when the input voltage Vin drops below the value of the output voltage Vout. Transistor Q11 then operates in the reverse direction, that is, a current I11 flows from the output to the input terminal OUT, IN. This “reverse current” I11 is a function of the voltage difference Vdiff between the terminals—the resistance value of resistance R11 and the current amplification factor of transistor Q11. The applicable equation for the reverse current is:I11=iB11·(βinv+1)=(Vdiff−Vth_inv)/R11·(βinv+1)  (1)
Here, iB11 denotes the base current flowing through resistance R11, Vdiff denotes the difference between output and input voltage Vout, Vin, and βinv denotes the current amplification of the transistor for operation in the reverse direction. Vth_inv denotes the threshold voltage of transistor Q11 operated in the reverse direction.
For applications in which the output capacitor Cout is supposed to store its charge for as long as possible in response to a loss or turn-off of input voltage Vin, this reverse current I11 is undesirable.
To reduce the reverse current, an approach is known from EP a 374 288 B1 in which, in addition to the first transistor connected between the input terminal and output terminal, a second transistor is provided which together with the first transistor forms a current mirror.
A circuit arrangement of this type is illustrated in FIG. 2. The second transistor in FIG. 2 is identified as Q12. This transistor Q12 is interconnected as a diode, and is connected between the output terminal OUT and the base of first transistor Q11.
For this circuit, the applicable equation for reverse current I11 is:I11=I12·(k+1)=(Vdiff−Vth12)/R11·k  (2).
Here Vth12 denotes the threshold voltage of second transistor Q12, while k denotes the current mirror factor between the two transistors Q12 and Q11. This current mirror factor k can be set such than it is smaller that the current amplification Binv of first transistor Q11, with the result that a smaller reverse current is produced in comparison with the circuit of FIG. 1.
Therefore, it would be advantageous to provide a circuit arrangement with a transistor connected between an input terminal and an output terminal, which transistor has a reduced reverse current in response to a drop in the input voltage below the value of the output voltage.